1. Field of the Invention
The invention relates to digital telecommunications. More particularly, the invention relates to methods and apparatus for detecting alarm indication signals when such signals are overwritten by other special codes.
2. State of the Art
The first commercial digital voice communications system was installed in 1962 in Chicago, Ill. The system was called xe2x80x9cT1xe2x80x9d and was based on the time division multiplexing (TDM) of twenty-four telephone calls on two twisted wire pairs. The digital bit rate of the T1 system was 1.544 Mbit/sec (xc2x1200 bps), which was, in the nineteen sixties, about the highest data rate that could be supported by a twisted wire pair for a distance of approximately one mile. The cables carrying the T1 signals were buried underground and were accessible via manholes, which were, at that time in Chicago, spaced approximately one mile apart. Thus, analog amplifiers with digital repeaters were conveniently located at intervals of approximately one mile.
The T1 system is still widely used today and forms a basic building block for higher capacity communication systems including T3 which transports twenty-eight T1 signals. The designation T1 was originally coined to describe a particular type of carrier equipment. Today T1 is often used to refer to a carrier system, a data rate, and various multiplexing and framing conventions. While it is more accurate to use the designation xe2x80x9cDS1xe2x80x9d when referring to the multiplexed digital signal carried by the T1 carrier, the designations DS1 and T1 are often used interchangeably. Today, T1/DS1 systems still have a data rate of 1.544 Mbit/sec and support typically twenty-four voice and/or data DS0 channels. Similarly, the designations DS2 and T2 both refer to a system transporting up to four DS1 signals (96 DS0 channels) and the designations DS3 and T3 both refer to a system transporting up to seven DS2 signals (672 DS0 channels). The timing tolerance for modern T1 equipment has been raised to xc2x150 bps.
The most recent standardized specifications for T1/DS1 systems are contained in several published standards including ANSI T1.102, ANSI T1.231, ANSI T1.403 and ITU-T Recommendation Q.921, the complete disclosures of which are hereby incorporated herein by reference. It is worth noting that the T1 system is substantially the same in North America and Japan but is different in Europe where it is known as xe2x80x9cE1xe2x80x9d, has a data rate of 2.048 Mbit/sec and multiplexes up to thirty voice and/or data channels.
The current standard for T1/DS1 systems incorporates many improvements and enhancements over the original T1 system. The basic T1 system is based on a frame of 193 bits, i.e. twenty-four 8-bit channels (the payload) and one framing bit (F). According to today""s standards, the 192 bit payload need not be xe2x80x9cchannelizedxe2x80x9d into 24 DS0 channels. In addition, superframe and extended superframe formats have been defined. The superframe (SF) format is composed of twelve consecutive T1 frames, i.e. approximately 1.5 milliseconds of a T1 signal. In the SF format, the twelve framing bits F are divided into two groups, six terminal framing bits Ft and six signalling framing bits Fs. The Ft bits are used to identify frame boundaries and the Fs bits are used to identify superframe boundaries. When the frames are DS0 channelized, the Fs bits are also used to identify signalling frames. The extended superframe (ESF) format is composed of twenty-four consecutive T1 frames, i.e., approximately 3 milliseconds of a T1 signal. In the ESF format, the twenty-four F bits are divided into three groups. Six F bits are used to provide a 2 kbps framing pattern sequence (FPS) which is used to identify the frame and ESF boundaries. When the frames are DS0 channelized, the FPS is to identify signalling frames. Another six of the F bits are used to provide a 2 kbps CRC (cyclic redundancy check-error checking) channel utilizing a CRC-6 code. The remaining twelve F bits are used to provide a 4 kbps data link (DL) channel.
In addition to modern framing conventions, the present T1 specification also includes provisions for different xe2x80x9cline codesxe2x80x9d, sometimes referred to as xe2x80x9ctransmission codesxe2x80x9d. It will be appreciated that the T1 signal is a plesiochronous (tightly controlled asynchronous) signal and, unlike a synchronous signal, is still subject to wander, jitter, and slips. Line codes are signalling conventions which are designed to facilitate frame synchronization and error detection. One popular line code is known generally as alternate mark inversion (AMI or bipolar line code). AMI utilizes a ternary signal (positive, negative, and null) to convey binary digits (zero and one). Successive binary ones are represented by signal elements of alternate polarity and of equal magnitude. Binary zeros are represented by signal elements having zero amplitude. Under the AMI line code, a non-zero signal element which follows a non-zero signal element of the same polarity is called a xe2x80x9cbipolar violationxe2x80x9d.
The T1 signal is also conditioned by pulse density requirements, i.e. the minimum number of xe2x80x9conesxe2x80x9d (marks or pulses) which must be present in given number of binary digits or xe2x80x9ctime slotsxe2x80x9d. Pulse density requirements prevent a lost signal from being mistaken for a long string of zero bits. An enhancement to the basic AMI line code which helps meet pulse density requirements is known as xe2x80x9cbipolar with 8-zero substitutionxe2x80x9d (B8ZS). The B8ZS line code provides that blocks of eight consecutive zeros are replaced with a unique eight bit code, i.e. 000VB0VB, where B represents a non-zero signal element conforming to the bipolar rule and V represents a bipolar violation. Another system for meeting pulse density requirements is known as xe2x80x9czero-byte time slot interchangexe2x80x9d (ZBTSI). According to ZBTSI, eight consecutive zeros are replaced by an address chain that is decoded by the receiving terminal. As mentioned above, these transmission codes are based on the nature of the T1 carrier and not on the DS1 multiplexing scheme. Today, a DS1 transmission path may be provided on media other than a T1 carrier. A DS1 transmission path which is synchronous (e.g. via SONET) and does not utilize line codes or pulse density requirements is said to have xe2x80x9cclear channel capabilityxe2x80x9d.
The present standards for SF and ESF formats provide means for sending maintenance signals. Exemplary maintenance signals include Remote Alarm Indication (RAI, or xe2x80x9cyellow alarmxe2x80x9d), Alarm Indication Signal (AIS), and, more recently, trouble sectionalization signals (RAI-CI and AIS-CI) which identify whether trouble exists at the customer installation (CI) or in the network. Other maintenance signals include loopbacks and loopback control signals. In the SF format maintenance signals are transmitted in-band (in one or more DS1 channels or in a T1 frame). In the ESF format; maintenance signals are transmitted in the DL channel.
The RAI signal is transmitted in the outgoing direction when DS1 terminal equipment located in either the network or the customer installation has effectively lost the incoming signal. The detailed requirements for sending an RAI signal are contained in previously incorporated ANSI T1.231. An RAI is transmitted to the NI in several forms. In the SF format, for the duration of the alarm condition, but for at least one second, bit two in every channel time-slot shall be a zero. In the ESF format, for the duration of the alarm condition, but for at least one second, a repeating 16-bit pattern of eight xe2x80x9conesxe2x80x9d followed by eight xe2x80x9czerosxe2x80x9d is transmitted continuously on the ESF DL channel, but may be interrupted for a period of 100 milliseconds per interruption for xe2x80x9cbit patterned messagesxe2x80x9d. Bit patterned messages are preemptive messages which will overwrite other signals in the DL channel.
The AIS maintenance signal (also known as a blue alarm) is transmitted in place of the normal T1 signal under certain specified conditions such as when an equipment experiences a loss of signal (LOS) at its input or is being placed in a maintenance state such as a loopback. The AIS signal is designed to be readily recognized by all equipment regardless of the framing or line codes employed and may be inserted by the CI or any element in the network. The AIS signal defined in previously incorporated in ANSI T1.231 is a signal having a ones density of 99.9% for a period xe2x89xa7T, where 3 millisecondsxe2x89xa6Txe2x89xa675 ms. The minimum time of 3 milliseconds was chosen so that an AIS which was corrupted by a bit error ratio (BER) of up to 1xc3x9710xe2x88x923 could be differentiated from a normal (framed) signal having a payload of all ones.
The recently defined trouble sectionalization signals (RAI-CI and AIS-CI) identify whether trouble exists at the customer installation (CI) or the network. More particularly, the RAI-CI signal is a repetitive pattern with a period of 1.08 seconds. It is formed by sequentially interleaving 0.99 seconds of the RAI (ESF) signal with 90 milliseconds of a xe2x80x9cbit patterned messagexe2x80x9d, i.e. eight xe2x80x9conesxe2x80x9d followed by one xe2x80x9czeroxe2x80x9d, followed by five xe2x80x9conesxe2x80x9d, followed by two xe2x80x9czerosxe2x80x9d. The RAI-CI signal may only be used in the ESF format. The AIS-CI signal is a repetitive pattern with a period of 1.26 seconds. It is formed by sequentially interleaving 1.11 seconds of an unframed all ones pattern (the AIS signal) with 0.15 seconds of a modified all ones pattern. The AIS-CI signal is defined as a pattern which recurs at 386 bit intervals in the DS1 signal. In other words, each 386th bit of an AIS signal is overwritten by another repetitive pattern, i.e. eight xe2x80x9conesxe2x80x9d, followed by two xe2x80x9czerosxe2x80x9d, followed by five xe2x80x9conesxe2x80x9d, followed by one xe2x80x9czeroxe2x80x9d. The AIS-CI pattern therefore repeats once every 6176 bits and differs from an ordinary AIS pattern in that bit numbers 3088, 3474, and 5790 are xe2x80x9czerosxe2x80x9d rather than xe2x80x9conesxe2x80x9d. The AIS-CI signal, like the AIS signal, has a ones density greater than 99.9%. The AIS-CI signal may be used in any of the T1 frame formats.
Those skilled in the art will appreciate that the AIS-CI signal is difficult to detect in the presence of line errors. It will be recalled that the AIS signal was designed to be detectable in the presence of a bit error ratio up to 1xc3x9710xe2x88x923. The AIS-CI pattern modifies the AIS pattern by introducing three zeros every 4 milliseconds (the CI code word) in the AIS pattern. It will be appreciated that AIS detection involves validating the xe2x80x9call-ones densityxe2x80x9d over a period of three to seventy-five milliseconds before declaring AIS. This is typically done by integrating the number of xe2x80x9czerosxe2x80x9d over a period of time and then checking that the number of xe2x80x9czerosxe2x80x9d is less than a preset threshold. When the AIS-CI code is present, the additional xe2x80x9czerosxe2x80x9d added to the AIS signal may be misinterpreted as indicative of the absence of an AIS signal. If the zero threshold value is adjusted to accommodate the presence of three additional zeros every 4 milliseconds (the CI code word), false AIS indications may result on a noisy line where a framed all ones (non-AIS) signal is present.
It is therefore an object of the invention to provide a method and apparatus for detecting both AIS and AIS-CI signals in the presence of a bit error ratio up to 1xc3x9710xe2x88x923.
It is also an object of the invention to provide a method and apparatus for detecting both AIS and AIS-CI signals which reliably detects both AIS and AIS-CI signals in the presence of bit patterned messages which overwrite the signals.
In accord with these objects which will be discussed in detail below, the apparatus of the present invention includes an AIS detector having an AIS indication output, a CI detector having a CI indication output, and a two signal AND gate having its inputs coupled to the respective outputs of the detectors and having an output indicative of an AIS-CI detection. The AIS detector has a zero threshold which is adjustable and the CI detector has a threshold output coupled to the AIS detector for adjusting the zero threshold of the AIS detector. According to the method of the invention, the AIS detector zero threshold is normally set at the normal AIS zero threshold (1xc3x9710xe2x88x923) but is reset to a higher threshold (e.g., 2xc3x9710xe2x88x923) when the CI detector detects the presence of the CI code word. According to the invention, the AIS signal is detected in a normal way and the AIS-CI signal is detected only when the CI detector detects the presence of the CI code word AND the AIS detector detects the presence of the AIS signal using the higher zero threshold. According to the presently preferred embodiment, the zero threshold of the AIS detector is raised if the CI detector detects two consecutive CI code words (i.e., the CI code for 8 ms), but the CI indication output is indicated only if the CI detector detects eight CI code words in 40 ms. According to the presently preferred embodiment, the zero threshold of the AIS detector is returned to normal when the CI detector detects ten consecutive absences of the CI code word (i.e., 40 ms without the CI code).
Additional objects and advantages of the invention will become apparent to those skilled in the art upon reference to the detailed description taken in conjunction with the provided figures.